Synchronizers for use in multi-ratio transmissions are well known. Such synchronizers may be of the double acting type including pairs of friction and jaw members for respectively synchronizing and positive clutching gears to a shaft, synchronizer assemblies for engaging the friction members in response to initial engaging movement of a shift sleeve, a hub rotatably fixed to the shaft and having external spline teeth slidably receiving internal spline teeth of the shift sleeve which often defines one of the jaw member pairs, a baulking means having blocker teeth for arresting engaging movement of the shift sleeve until synchronization is reached and for transferring a shift force from the sleeve to increase the engaging force of the friction members.
It is also known that such synchronizers may be used to reduce shift time and shift effort required by a vehicle operator by the providing the synchronizers with self-energizing means. EP-A-0663 541 discloses a synchronizer having all of the above.